They can be fine if stocked and maintained properly - otherwise they're a pain (stocking and maintainance can be a pain in itself IME) The rectangular ones are probably better than the round ones though IMHO.
IMHO...
Pros:
Some may find them more attractive than regular rectangular tanks
Good range of fake decor for them (but it is pretty pricey)
Cons:
The filtration isn't too great
They can't be stocked the same as regular rectangular tanks
The lighting isn't the best
You're stuck with ceramic media
Less surface area than regular rectangular tanks
Very expensive for what they are
The 'orbs' are little more than glorified fish bowls
Acrylic scratches easily
Dirty easily
Maintainance is a pain
Can't really plant them very much (substrate, undergravel filtration and size/shape/filter tube makes it harder to have bogwood for anubias, jara fern, etc)
IME, the pump isn't terribly quiet, though not unbearably noisy either
I've had a BiOrb myself and truly regret it looking back. My betta is so much happier in his rectangular tank than he was in his biorb and it was cheaper too. So much easier to maintain, far better lighting, I can plant it, easier to decorate, can have different substrates, easier and more flexible to stock, and so it continues...